1. Statement of the Technical Field
The invention concerns thermal energy cycles, and more particularly systems and methods for merging thermal energy cycles including multi-pass energy recirculation techniques which enable normally rejected thermal energy to be re-used in the cycle, repeatedly.
2. Description of the Related Art
Heat engines use energy provided in the form of heat to perform mechanical work, and exhaust a portion of the applied heat which cannot be used to perform work. This conversion of heat energy to mechanical work is performed by taking advantage of a temperature differential that exists between a hot “source” and a cold “sink.” This well known concept is illustrated in FIG. 1 which shows a hot reservoir, a cold reservoir and an intermediate thermodynamic cycle which generates work as an output. Heat engines can be modeled on various different well known thermodynamic processes or cycles. Two such well known heat engine cycles include the Brayton cycle and the Rankine cycle.
The closed Brayton cycle is shown in FIG. 2. A working fluid is pressurized in a compressor 202 which performs work (Win2) and then heated (Qin2) by heat source 204. The heated pressurized working fluid then releases energy by expanding through a turbine 206. A portion of the work (Wout2)which is extracted from the heated and pressurized working fluid by the turbine 206 is used for driving the compressor 202. The working fluid is then cooled (Qout2) in a cooler 208 and the cycle is repeated.
The foregoing example is one in which the Brayton cycle is run as a closed cycle. The Brayton cycle can also be run as an open cycle (open Brayton cycle). In such an arrangement, ambient air is drawn into a compressor, where it is pressurized. The compressed air is communicated to a combustion chamber where fuel is burned and the compressed air is heated in an isobaric process (i.e. at constant pressure). The heated and pressurized air is thereafter caused to expand through a turbine where mechanical work is produced. Some of this work is used to drive the compressor. The air is thereafter permitted to exhaust from the turbine into the ambient environment (the atmosphere). Gas turbine engines examples of open Brayton cycles and are common on aircraft and in power plants.
FIG. 3 illustrates the basic features of the Rankine cycle. In the Rankine cycle, a liquid working fluid is pumped from low to high pressure by a pump 302, thereby adding work to the system (Win3). The pressurized working fluid is then passed to a boiler 304 where it is heated (Qin3) at constant pressure by a suitable heat source to become a vapor. The vapor thereafter expands through an expander or turbine 306, providing work (Wout3) as an output. This process of expanding through the turbine results in a decrease in pressure and temperature for the vapor, and may include some condensation. The vapor and condensation are then passed to a condenser 308 where the vapor is condensed or cooled to remove heat (Qout3) at a constant pressure to become a liquid. The liquid is then passed to the pump, after which the process is repeated.
A combined cycle is an assembly of two or more engines that convert heat into mechanical energy by combining two or more thermodynamic cycles. The exhaust of one heat engine associated with a first cycle is used to provide the heat source that is used in a second cycle. For example, an open Brayton cycle is commonly combined with a Rankine cycle to form a combined cycle for power plant applications. The open Brayton cycle is typically implemented as a turbine burning a fuel, and the exhaust from this combustion process is used as the heat source in the Rankine cycle. In such a scenario, the Rankine cycle is referred to as a bottoming cycle because it uses some waste heat from the Brayton cycle to perform useful work. When using high temperature sources of heat (e.g. 2000° F.), a combined open Brayton cycle with a Rankine bottoming cycle can ideally be expected to provide an energy conversion efficiency as high as 60% . In the case of low temperature heat sources (e.g. 700° F.) conversion efficiencies are much lower, traditionally below about 35%.